1. Field of the Invention
This invention relates to stabilized thermoplastic, segmented copolyesters, to blends of such stabilized copolyesters with one or more compatible low molecular weight thermoplastic resins and to a method for preparing such blends.
2. Description of the Prior Art
Copolyesters, and particularly segmented copolyesters, are used in the formulation of adhesives such as those useful as hot melt adhesives. Such adhesive compositions are described in U.S. Pat. No. 3,832,314 and Hoh and Reardon Ser. No. 439,848, filed Feb. 6, 1974. The compositions of U.S. Pat. No. 3,832,314 have good bond strength as hot melt adhesives and the compositions of the Hoh and Reardon application are particularly useful as pressure sensitive adhesives. In order to provide good adhesive properties the viscosity of the adhesive compositions must be maintained at a relatively constant level. It has been found that at elevated temperatures, particularly in the range of 170.degree. to 200.degree. C., over a period of several hours the aforementioned adhesive compositions lose viscosity and hence their bonding properties are reduced. Known stabilizers or antioxidants such as tetrakis[methylene-3-(3',5'-ditertiary-butyl-4'-hydroxyphenyl) propionate] methane, and phosphite ester compounds, have been incorporated in hot melt adhesive compositions containing segmented copolyesters, but these stabilizers have not proved to be very effective over extended periods of time, e.g., two hours and more.
Materially improved stabilization of adhesive compositions containing segmented copolyesters has been achieved as described in U.S. Pat. No. 3,909,333 utilizing a stabilizer mixture comprising (a) a substantially linear polycarbodiimide having an average of at least two carbodiimide groups per molecule, and at least one compound taken from the group consisting of (b) a hindered phenol, nitrogen-containing hindered phenol, or secondary aromatic amine; (c) phosphorous acid esters of the formula ##STR1## where R.sub.1, R.sub.2 and R.sub.3 are C.sub.1 to C.sub.18 aliphatic, C.sub.6 to C.sub.15 aromatic, and combinations thereof; and (d) a homopolymer of an amino acrylate compound of the formula ##STR2## where R is hydrogen or methyl, R.sup.1 is hydrogen or alkyl of 1 to 4 carbon atoms, R.sup.2 is alkyl of 1 to 4 carbon atoms, and n is an integer of 1 to 4 inclusive; and a random copolymer of ethylene and 20 to 40 percent by weight of the amino acrylate, each of the compounds (b), (c) and (d) being compatible with each other and with (a). While providing good stabilization of segmented copolyester compositions, the stabilizer mixture of U.S. Pat. No. 3,909,333 is relatively expensive and may create compatibility problems in some adhesive formulations. It was therefore desirable to discover a single stabilizer compound which is compatible in conjunction with other components and not only provide excellent stabilization but is inexpensive and is also compatible with known segmented copolyester adhesive compositions.